Method and apparatus for optically powering and multiplexing distributed fiber optic sensors

ABSTRACT

An optical power converting apparatus is provided that may be used with remote sensors. A plurality of remote sensors may be coupled to a backbone optical fiber with each sensor having an optical power converter that receives an optical signal from a head end of that fiber. The optical power converters may store electrical energy derived from that optical signal and use that energy to power the remote sensors. The head end&#39;s optical signal may also include a clock signal, and each remote sensor may be set to sense a measurable parameter after a given number of clock cycles have been counted. In a further example, each of the optical sensors may be synchronized before counting these clock signals via a synchronization signal from the optical power converter. The remote sensors may individually and separately uplink their sensed data to the head end on the optical fiber. The apparatus may be implemented in a vehicle health management system, for example.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to and is a divisional of U.S. patentapplication Ser. No. 11/079,688 filed Mar. 14, 2005 now U.S. Pat. No.7,263,245, which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to sensors and more particularlyto optically-powered sensors.

BACKGROUND OF THE RELATED ART

For years, people have used remote sensors in hostile environments,placing sensors in locations where human or manual data collection isunattainable or too costly. In toxic and environmentally dangerousenvironments, for example, remote sensors may provide an effective wayof measuring data that might otherwise go unmeasured. Inspace-constrained environments, remote sensors may be useful in reachingotherwise unreachable locations.

Despite the deployment of remote sensors in certain applications, manyapplications are incompatible with certain types of remote sensors, orremote sensors altogether. Electrically-powered remote sensors, forexample, are not used in environments where electrical conduction canlead to sensor damage or environmental damage. In an aircraft, forexample, lightning strikes can be dangerous and damage on-boardelectronics such as those that would be used in and to power electronicsensors. This lightning problem may be exacerbated by the light-weight,less shielding composite structures used with increasing regularity inmodern aircraft. In a spacecraft, for example, a lightning strike couldharm the electronic sensors used to monitor mission critical launchconditions. In fact, lightning damage has resulted in some infamouslosses of spacecraft, including the Atlas G-Centaur AC-67 space mission.Lightning also nearly caused the astronauts to abort the Apollo 12spacecraft launch, when a lightning strike triggered electrical warningsignals and disabled telemetry systems. Moreover, the problem of spikingis not limited to lightning, as other high voltages would be hazardousif combined with electrically powered sensors in certain environments,such as inside fuel tanks where there is the potential for an ignitinghazard through short circuits in the electrical wiring.

Not only are electrical field and voltage surges problematic, high-powermicrowave radiation can also limit the use of certain types of sensors.For example, it is difficult to use electronic sensors to monitorhigh-power phased array radar systems because of electromagnetic fieldinterference. High voltage isolation is a limiting factor forhigh-voltage, power-line sensor applications, as well.

Whereas electrically-powered sensors may be incompatible with certainenvironments, optically-powered sensors may show potential. In aircraft,for example, an optically powered sensor could protect againstlightning, electric fields and discharges, and other electronicinterference.

Yet, despite the theoretical attractiveness of optically-poweredsensors, there are numerous limitations affecting their deployment. Oneproblem is the lack of efficient and effective methods to opticallypower multiple sensors. Some powering techniques convert an opticalenergy on a fiber to electrical power at the sensor. However, thetechniques are only used to power a single sensor, unless a fiber opticsplitter or multiplexing device is used, thereby adding to device cost,weight, and complexity. Furthermore, remote powering techniques canrequire a minimum of two fibers for each sensor—one fiber to opticallypower the sensor, another fiber to receive sensor data. Even thecommercially-available pie-wedge photonic power converters suggested bysome (in addition to being expensive) would require a fiber bundle toreceive data from multiple sensors. In short, the present techniques foroptically powering remote sensors would require multiple fibers or alarge fiber bundle if multiple sensors were to be deployed, and thisrequirement is undesirable in space—or weight-constrained systems suchas an aircraft, or spacecraft.

It is desirable to have a way of optically powering multiple sensorsthat may be placed remotely from one another, and to do so in a way thatremote sensed signals may be communicated to a centralized analyzer viathe same fibers used for powering the sensors.

SUMMARY OF THE INVENTION

An embodiment of the invention is an optical power converter comprising:a photodetector for producing an electrical signal; a storage circuit inparallel with the photodetector to store at least a portion of theelectrical signal; a first chargeable switch; a second chargeable switchhaving a different charging time than the first chargeable switch; and adual transistor switch coupled to the first chargeable switch and thesecond chargeable switch, wherein the storage circuit is coupled to thedual transistor switch, and wherein during a storing state thephotodetector supplies current to the storage circuit and the dualtransistor switch is in an off state, and wherein during a drivingstate, the storage circuit supplies current to switch the dualtransistor switch to an on state wherein at least one of the firstchargeable switch or the second chargeable switch is in a conductingstate.

Another embodiment of the invention is an optically-powered sensorapparatus comprising: an optical fiber; a head end coupled to theoptical fiber to provide optical clock signals on the optical fiber; andat least two sensor modules coupled to the optical fiber and opticallypowered by the optical clock signals, each sensor module comprising anoptical power converter for converting the optical clock signals toelectrical clock signals, a timer for counting the electrical clocksignals, and a sensor for sensing a measurable parameter, wherein the atleast two sensor modules are adapted to sense the measurable parameterafter a different number of electrical clock signals have been counted.

A further embodiment of the invention includes a method of time divisionmultiplexing a plurality of sensor modules coupled to an optical fiber,the method comprising: transmitting an optical signal on the opticalfiber, the optical signal having a clocking portion wherein opticalclock signals are provided and a synchronizing portion; at each of theplurality of sensor modules, receiving the optical signal and convertingthe optical clock signals of the clocking portion to electrical clocksignals; synchronizing each of the plurality of sensor modules; countingthe electrical clock signals; and for at least two of the plurality ofsensor modules, sensing a measurable parameter after a different numberof electrical clock signals have been counted.

Another embodiment of the invention includes an optically-powered sensorapparatus comprising: an optical fiber; a laser source coupled to theoptical fiber for providing optical clock signals on the optical fiber;a first sensor module coupled to the optical fiber and optically poweredby the laser source, the first sensor module having a sleep mode duringwhich the first sensor module is incapable of sensing a first measurableparameter and an awake mode during which the first sensor module iscapable of sensing the first measurable parameter, wherein the firstsensor module is adapted to switch from the sleep mode to the awake modeafter a first number of optical clock signals have been received at thefirst sensor module; and a second sensor module coupled to the opticalfiber and optically powered by the laser source, the second sensormodule having a sleep mode during which the second sensor module isincapable of sensing the second measurable parameter and an awake modeduring which the second sensor module is capable of sensing the secondmeasurable parameter, wherein the second sensor module is adapted toswitch from the sleep mode to the awake mode after a second number ofoptical clock signals have been received at the second sensor module,where the second number of optical clock signals is different than thefirst number of optical clock signals.

Another embodiment of the invention includes a method of diagnosing thestate of a vehicle, the method comprising: coupling optical clocksignals to the plurality of sensor modules via an optical fiber, eachsensor module being disposed at a region of interest and each sensormodule having a sleep mode and an awake mode; optically powering theplurality of sensor modules; at each sensor module, counting the numberof optical clock signals received during the sleep mode; at each sensormodule, in response to the counting of the number of optical clocksignals received during the sleep mode, switching the sensor module fromthe sleep mode to the awake mode, where each sensor module is switchedfrom the sleep mode to the awake mode after a different number ofoptical clock signals have been counted; at each sensor module, sensinga measurable parameter and producing sensed data; and diagnosing thesensed data from each sensor module.

Some of the embodiments of the invention provide devices and techniquesthat fiber optically power multiple sensors on an optical fiber. Theprinciple of operation of the optically powered distributed sensors canvary, according to the parameters being sensed, and system designerpreference. For example, such sensor nodes can be electronic, magnetic,optical, electro-optic, acoustic/ultrasonic, or combinations thereof. Insome of these examples, these sensors may be time division multiplexedto communicate a sensed signal on the same optical fiber used to deliverpower to the sensor. The devices and techniques may includesynchronizing the multiple sensors and having the multiple sensorscommunicate their sensed signals on the optical fiber, withoutinterfering with an optical powering signal on that fiber. Of course,the features, functions, and advantages can be achieved independently invarious embodiments of the present invention or may be combined in yetother embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of an optical sensor system having a headend and a plurality of sensor modules coupled to an optical fiberextending from the head end.

FIG. 2A illustrates a detailed example of a sensor module that may beused in the system of FIG. 1.

FIG. 2B illustrates an example of direct coupling of a photodetector tothe side of an optical fiber, using a small slanted notched surfacecreated at the side of the fiber, to improve light tapping efficiency.

FIGS. 3A and 3B illustrate a detailed example of a head end that may beused in the system of FIG. 1.

FIG. 3C illustrates an optical slit and mirror plate that may be usedwith the head end of FIGS. 3A and 3B to separate the outgoing laserpower from the incoming data signals from the sensors.

FIG. 4 illustrates an example of a power converter that may be used tocouple optical energy from an optical fiber into a sensor on a sensormodule.

FIG. 5 illustrates a circuit diagram of an example implementation of thepower converter of FIG. 4.

FIG. 6 illustrates a block diagram of an example of a sensor apparatusthat may be coupled to the power converter of FIG. 5.

FIG. 7 illustrates a timing diagram of clock, synchronization, andoutput signals for an example of an optically powered sensor system.

FIG. 8 illustrates a timing diagram of synchronization and outputsignals for another example of an optically powered sensor system.

FIG. 9 illustrates an example aircraft environment with an opticalsensor system having a head end and a plurality of distributed sensormodules.

DETAILED DESCRIPTION OF AN EXAMPLE

Numerous exemplary devices and techniques are described below, and someare described in relation to rather detailed examples. However, thedevices and techniques are not limited to such examples, but rather maybe implemented in various applications. For example, although someexamples are described as usable in an aircraft, including spacecraft,the devices and techniques may be used in other vehicles or systems. Thedevices and techniques may be used, more broadly, in any environment inwhich remote sensing via an optical fiber and a head end, receiverstation, or transmitter station may be desired. The devices andtechniques may be used in various sensor applications, such as hydrogensensors applications, oxygen sensing applications, or strain sensorapplications, e.g., in shell composite layers. Yet, other applications,e.g., measuring performance along a high-voltage electrical transmissionline, will be known to persons of ordinary skill in the art uponreviewing the disclosure herein.

FIG. 1 illustrates an example of an optoelectronic sensor system 100that may be used to monitor conditions in remote environments, bothnon-hostile and hostile. Example environments include portions of anaircraft (e.g., an airplane or spacecraft) or other airborne vehicles(e.g., un-manned booster rockets). Further examples include environmentswhere there is potential for exposure to harsh or hazardous conditions,such as oil wells.

To provide remote sensing within a system, such as an aircraft, thesystem 100 has a head end 102, or base station, that communicates with aplurality of sensor modules 104, via an optical fiber backbone 106. Thehead end 102, for example, may have a laser source and may send anoptical signal on the fiber 106 to the sensor modules 104. As describedin further detail below, that optical signal may provide timinginformation to the sensor modules 104 and may also optically power them.Based on the timing information, the sensor modules 104 may providesensed data back to the head end 102.

Although the modules 104 may be placed in traditionallydifficult-to-reach locations, the head end 102 typically is stored in anon-hostile location accessible to personnel or analysis equipment.However, this need not be the case.

In the illustrated example, the backbone fiber 106 extends along aseries of monitoring regions 108 each representing an area monitored bythe modules 104, such as different areas within an aircraft. The sizesof the regions 108 may depend on the type of sensors deployed, and thus,are only generally shown. The apparatuses and methods described hereinare not limited to a particular type of sensor.

The backbone fiber 106 may be formed of any number of suitable opticalfibers. However, as certain remote environments may benefit from morerugged fibers, a hard clad silica (HCS) trunk fiber, such as a 200 μmcore, multimode fiber may be used. By way of example, not limitation, afiber rated at 40 pounds or higher tensile strength may be used. Anexample HCS fiber is available from OFS Specialty Photonics of Avon,Conn. Further, the fiber 106 may be coated with a plastic coating thatprevents moisture from producing embrittlement within the fiber core,which would reduce fiber strength. However, despite these examples, thepresent disclosure is not limited to a particular fiber, size,composition, or fabrication technique.

Each sensor module 104 may be coupled to the fiber 106 via a coupler 110and each sensor module 104 may have at least one sensor 112 coupled tothat coupler 110. The coupler 110 may represent two couplings, one acoupling of the module 104 to the fiber 106 for receiving optical power,the other a coupling of the sensor module 104 to the fiber 106 fortransmitting a sensed signal on the fiber 106. The couplings may beachieved through a variety of techniques, such as hard clad silicatapping techniques. For example, to receive optical power, a photodiodemay be directly adhered to the fiber 106 to absorb the optical powerfrom the fiber 106. In this case, the efficiency of coupling the opticalpower to the photodiode may be enhanced by removing part of the fibercladding. Removing the cladding at an angle, to create a slanted surfacecan reflect light more efficiently onto the photodiode attached to theside of the fiber. For improved efficiency, the surface of the slant canbe metallized, for example, by metal evaporation. An example couplingtechnique is shown in FIG. 2B discussed below. To transmit a sensedsignal, a tap optical fiber (e.g., optical fiber tap 204, in FIG. 2A)may be coupled to the fiber 106. If the sensor 112 includes a verticalcavity surface emitting laser (VCSEL), for example, the tap fiber at thesensor 112 may be positioned at the focal point of a lens that collectsthe output from that VCSEL. A VCSEL produces an emission cone that istypically quite small, and, thus, a small tap may be used, one that isnot susceptible to detrimental power leakage into the VCSEL from theoptical power on the fiber 106. In another example, the sensed signalmay be coupled directly from the VCSEL into the fiber 106, without useof a tap fiber or waveguide, in a similar way as discussed for thephotodetector. In another example, the coupler 110 may include a singletap fiber in combination with a splitter/combiner for coupling receivedand transmitted light.

The couplings of coupler 110 may be designed to occur at a radial bendof the fiber 106, where the bend will naturally facilitate light leakageinto or out of the fiber. The cladding of the fiber 106 at the bend maybe partially or fully removed by an etching or ablation technique toenhance coupling efficiently.

FIG. 2A illustrates a more detailed example of the sensing module 104and, thus, shares like reference numerous with FIG. 1. A photodetector200, or solar cell, is coupled directly to the fiber 106 to receiveoptical power signals. The photodetector 200 is also connected to apower converter 202, for optically powering the sensor 112, as explainedin further detail below.

In the illustrated example, an optical fiber tap 204 is coupled directlyto the sensor 112 to couple optical energy from the sensor 112 into thefiber 106, for example to upload a sensed output signal to the head end102. Each of the sensor modules 104 of FIG. 1 may be identical to thatdetailed in FIG. 2A, or some or all of the modules 104 may be different.

FIG. 2B illustrates an example of direct coupling of a photodetector 200to the side of the optical fiber 106, thus extracting some light leakingthrough the fiber cladding at the location of the contact, which may usean adhesive 216. Light extraction efficiency into the photodetector 200can be significantly increased using, for example, a small notch 210 atthe side of the fiber. The intersection of the angled surfaces 212 and214, which constitute notch 210 need to be rounded and not at a sharpline, so the notch does not weaken the fiber 106 significantly. Fiber106 may be glued down locally near the location of the notch to a smallsupport plate 218, to additionally strengthen the fiber 106, near thelocation of the notch. Notch 210 may be created by, for example, gentlyrunning a file with triangular cross section against the side of thefiber. A small fraction of the light traveling in fiber 106 exits theapproximately vertical surface 212, and reflects from the slantedsurface 214 onto the photodetector 200. The surface 214 can be made morereflective by deposition of a thin metal film, for example, by angleddeposition, such that the surface 212 is not metallized.

A detailed example of the head end 102 is shown in FIGS. 3A and 3B. Thehead end 102 may include two sections, a coupler 300 and a laser module302, where the fiber 106 is connected to the coupler 300 via a pigtailconnection 304, in the illustrated example.

The coupler 300 provides spatial filtering to transmit optical powersignals from laser 314 part of the head end 102 into the fiber backbone106 to the sensor modules 104, and to transmit sensed optical signalsfrom the sensor modules 104 to a head end photodetector (e.g.,photodetector 320) for receiving sensor data. The coupler 300 includesan optical plate 306 having a slit 308 (see, FIG. 3C). The plate 306 maybe coated with reflective material, such as aluminum, silver, chrome orgold, or made of such reflective materials. An example plate 306 is theNT38-559 precision air slit available from Edmund Industrial Optics ofBarrington, N.J. The plate 306 is positioned a distance from an inputlens 310, positioned a focal distance from the pigtail 304. The opticalplate 306 is also positioned a distance from a lens 312 coupled to alaser source 314 for producing the optical power signals. The lasersource 314 may be one that emits output energy over a strip width, forexample, over a 5 μm×50 μm output window. The slit 308 may be sized andpositioned such that the output from the laser source 314 may be coupleddirectly through the slit 308 and into the fiber 106, through lenses 310and 312, as shown.

The optical plate 306 is also positioned to communicate with a lens 316,which receives optical signals from the remote sensors via the fiber106, after energy from the fiber 106 is reflected by the plate 306 ontoan optional mirror 318. That is, for light coming out of the fiber 106,the end of the fiber 106 is imaged onto the slit 308. But if the fiber106 is circular in cross-section at its end, and if the image of thatcore is large enough, then much of the light signal imaged on the plate306, and centered on the slit 308 will fall outside the area of the slit308, and reflect off the plate 306, and imaged by the lens 316 onto aphotodetector 320.

In an alternative example to spatial filtering, a dichroic beam splitteror color filtering may be used in the coupler 300, whereby the spatialfilter 306 is replaced by a filter that transmits the wavelength oflaser 314 at the head end 102, but reflects the wavelengths of all ofthe VCSELs or other light sources such as light emitting diodes atsensor modules 104. Alternatively, a prism or otherpolarization-dependent reflector may be used to transmit the power laser314 light and reflect light returning from the sensor modules 104.

The head end 102 also includes the module 302 that includes the powerlaser section and sensor data receiver section(s). The module 302includes the laser source 314 and the photodetector 320, as well as amicrocontroller 322 that controls operation of the laser source 314through a digital-to-analog converter (DAC) 323. The laser source 314may be any type of laser source, including an edge emitting laser,VCSEL, or diode laser. Alternatively, the laser source 314 may be achemical or gas laser, or may represent an optical amplifier, such as afiber amplifier or optical parametric amplifier.

In operation, the head end 102 may provide an optical signal having botha high state and a low state. Therefore, a modulator 324 is positionedto modulate the output from the laser 314. The optical signal, forexample, may include an optical clock signal. That is, the laser module302 may produce optical clock signals of any given repetition rate, forexample, a 50% duty cycle optical clock signal having a 1 to 10kilohertz repetition rate. In the illustrated example, the modulator 324receives a clock control signal from a clock circuit 326 coupled to themicrocontroller 322. Alternatively, the modulator 324 may be part of thelaser 314.

Contrastingly, to receive optical energy from the fiber 106, the outputof the photodetector 320 is provided to an amplifier 328, such as atransimpedance amplifier. The amplifier 328 is coupled to ananalog-to-digital converter (ADC) 330 coupled to the microcontroller322. Module 302 containing the laser and sensor data receiver sectionsmay be powered by a power supply 332.

In an example operation of the system 100, the head end 102 produces anoptical signal (e.g., one having clock signals over at least a portion)on the fiber 106 that propagates to each of the sensor modules 104. Thepower converters 202 at each module 104 may receive this optical signalat substantially the same time, to power the sensor modules 104. Theclock signal portion of the optical signal may provide a timing signalthrough which these modules 104 may be instructed to turn on and beginsensing. For example, each distinct module 104 may be set to sense ameasurable parameter after receipt of different numbers of these timingsignals. The modules 104 may then use the fiber 106 to uplink a sensedoutput signal or other signal to the head end 102, e.g., after theirrespective number of timing signals has been received and during a timeperiod the head end is not providing an optical signal thus avoidinginterference. As such, the system 100 may provide a time divisionmultiplexed set of remote sensors that are optically powered by a signalon the same fiber that is used to transmit (uplink) sensed signals fromthe remote sensors.

FIG. 4 illustrates an example power converter circuit 400 that may beused as the power converter 200. A photodetector 402 receives an opticalclock signal and converts that signal to electrical energy that isstored in an electrical storage device 404, in parallel with thedetector 402. An output from the detector 402 is also coupled to a firstchargeable switch 406 and a second chargeable switch 408. The firstchargeable switch 406 is connected to a first output line 410, which maybe a clock signal line. The second chargeable switch 408 may be coupledto a second line 412, which may be a synchronization (sync) signal line.Both chargeable switches 406 and 408 are coupled to a switch 414 coupledto ground. In an alternative example, two switches may be used, one foreach of the elements 406 and 408, and both switches may be coupled toreceive an output from the detector 402, which may be a photo-responsivedevice, such as a photodiode or solar cell. Example photodetectorsinclude PIN photodiodes and pie-wedge-type photodetectors, such as thePPC-6E available from Photonic Power Systems of Cupertino, Calif.Additional examples are provided herein, for example in connection withFIG. 5.

The chargeable switches 406 and 408 may be separately chargeable, suchthat when charged the switch 414 can put the charged switches 406 and408 into a conducting state. If either of the switches 406 and 408 isuncharged, then the switch 414 would not place that switch into aconducting state. The switch 414 has an on state and off state and maybe any electrically controllable switch, including a bipolar transistor,integrated gate bipolar transistor, field effect transistor includingJFETs or MOSFETs (which may be either enhancement or depletion modedevices), uni-junction or programmable uni-junction transistor, an SCR,Schottky diode, or any combination of these, which may be both discreteor integrated in form, and may or may not be matched in the sense thatthis term is applied within the field of differential amplifiers.

The electrical energy from the detector 402 is partially stored in theelectrical storage device 404 and is partially used to assist insaturating the chargeable switches 406 and 408. The detector 402 mayprovide an electrical signal during a high cycle of a clocking portionof the optical signal on fiber 106, i.e., when photons are received atthe detector 402, at which time the storage device 404 is in a storingstate. During the low cycle of a clocking portion of the optical signal,no electrical energy is produced by the detector 402, but instead, theelectrical storage device 404 enters a driving state and powers theswitch 414 to turn on the chargeable switches 406 and 408. The switch414 may turn on one or both of these switches 406 and 408, dependingupon the desired operation and upon whether the switches 406 and 408 arealready charged. For example, during normal clock signal operation, theswitch 414 may turn on only the chargeable switch 406, to ensure that aclock signal is communicated on the line 410. This could create anelectronic clock signal every optical clock cycle, for example. Theswitch 414 may turn on the chargeable switch 406 less frequently, forexample, to communicate a less-frequent sync signal on the line 412. Adetermination as to when to turn on either of the switches 406 or 408may be made by switch 414 or via the information in the optical signal.

FIG. 5 illustrates a detailed circuit 500 that represents an exampleimplementation of the power converter 202. The circuit 500 includes asolar cell 502 (as the detector 402) for receiving an optical clocksignal from a coupler connected to a backbone optical fiber, such as thefiber 106. The solar cell 502 is coupled across an inductor (X2) 504 anda capacitor (C2) 506 that form the storage device 404. These elementsare coupled to a power output line 507 and a common node 508. The node508 is also coupled to a first, low resistance resistor (R2) 510 and afirst, higher resistance resistor (R3) 512. The node 508 is also coupledto a second, low resistance resistor (R4) 514 and a second, higherresistance resistor (R1) 516. Resistors 510 and 512 are coupled across acapacitor (C1) 518, and resistors 514 and 516 are coupled across acapacitor (C3) 520. Capacitor 518 is coupled to a clock signal outputline 522 and to the collector of a transistor 524 at node 526. Capacitor520 is coupled to a synchronization signal output line 528 and to thecollector of a transistor 530 at node 532.

The transistors 524 and 530 are each coupled to ground at their emittersand share a base node 534. In this configuration, the transistors 524and 530 form a dual transistor switch 536 with a base node 534 coupledto the bypass capacitor 506. The dual transistor switch 536 may be aXN5553 transistor, available from Matsushita Corporation of Japan. Thesolar cell 502 may have a p-type/insulator/n-type (PIN) layerconfiguration, as these configurations have lower capacitancetranslating into a lower power level threshold. Various solar celldevices for optical powering may be used, including pie-wedge solarcells. By way of example, not limitation, solar cells may be formed of agallium arsenide (GaAs), gallium indium phosphide, aluminum galliumarsenide, indium gallium arsenide, silicon, germanium or a combinationof these. Multi-layer solar cell structures formed on a single wafer,such as a GaAs wafer, may be used. Further, the solar cell 502 may ormay not have an anti-reflection coating, or other measures to improveefficiency.

In operation, the solar cell 502 forces current to flow through theinductor 504 during the high cycle of the optical signal. During the lowcycle, no current is produced by the solar cell 502, but rather thesolar cell 502 is reverse biased, which blocks current flow compared tothe direction of current flow during the high cycle. In an exampleimplementation, the optical signal may include an optical clock signalhaving a 50% duty cycle and a repetition rate from about 1 to 10 kHz,resulting in high and low cycle times of between 0.05 to 0.5milliseconds.

During the low cycle of the optical signal, the inductor 504 reversebiases the solar cell 502, and voltage across the inductor 504 continuesto rise in an inductive kick until the inductor 504 changes from astoring state to a driving state and forces current to flow through thecapacitor 506 and into emitter-base junctions 538 of the transistorswitch 536. The emitter-base junctions 538 act as a rectifier for thecircuit 500. Using a XN5553 circuit as the switch 536, the switch 536 iswell matched and is reverse biased on the emitter-base voltage by thebypass capacitor 506. Thus, the dual transistor 536 has relatively lowleakage current. The dual transistor may also have a high voltage ratingon the reverse emitter-base voltage, in an example, 15 volts.

When base current flows from the inductor 504 into the transistor switch536, both transistors 524 and 530 turn on, pulling the collectors a bitbelow ground, because the emitters go below ground by a diode drop, andthe transistors 524 and 530 saturate. The saturation pulls nodes 526 and532 on capacitors 518 and 520 low, respectively. If these capacitors 518and 520 have had sufficient time to charge, through resistors 512 and516, then the clock and sync output lines 522 and 528, respectively,will pull low, as well.

The capacitance and resistance values for the circuit 500 may be setsuch that only the clock line 522 pulls low every clock cycle, however.The RC time constant of the resistors 514, 516 and capacitor 520 may belong enough to prevent a sync pulse from being sent on line 528 simplyfrom the high/low transistors of the optical clock portion of theoptical signal. That is, the charging time during the optical clockcycle will not be sufficient to charge this RC constant, with only a fewmilliseconds of charging from the solar cell 502. Instead, as explainedin further detail below, the head end may skip a number of optical clockcycles, within the optical signal, every few seconds. If the skippednumber of optical clock cycles is long enough, the capacitor 520 willsaturate and a sync signal will be provided on line 528 at the start ofthe next optical power pulse. By way of example, every 1 to 3 seconds,approximately 15 optical clock cycles in a row may be skipped on theoptical signal from the head end, resulting in about 15 milliseconds ofno signal. This may leave enough time to charge the capacitor 520through resistor 516.

Example values for various capacitors and resistors of the circuit 500are provided in Table 1. These values are by way of example only, as isthe structure of the circuit 500. The circuit elements illustrated maybe replaced or eliminated. The inductor 504 may be replaced with anotherstorage device, such as a transformer, for example.

TABLE 1 Element Example Values X2 .5 H C1 47 pf C2 100 μF C3 220 pF R144 MΩ R2 39 KΩ R3 2.4 MΩ R4 39 KΩ

The sync and clock lines 528 and 522 are coupled to a microcontroller600 of a sensor apparatus 602 (shown in FIG. 6). An example of amicrocontroller that may be used as the microcontroller 600 is the PICmicrocontroller, available from Microchip Technology of Chandler, Ariz.,which is capable of counting a signal (e.g., a clock signal) during asleep mode. Sleep-mode counting has the advantage of reduced powerconsumption, as the microcontroller only periodically awakes, forexample, after a given counter number has been reached.

As shown, the clock line 522 is coupled from the power converter 500 toa timer circuit 604 within the microcontroller 600. As the capacitor 518saturates every optical clock cycle, the power converter 500 provides anelectrical clock signal to the timer circuit 604 every optical clockcycle, which the timer 604 counts, in an example. The sync line 528 iscoupled to an interrupt circuit 606 of the microcontroller 600, whichmay be used to synchronize the sensor 602 with other sensors on thebackbone fiber, so that each sensor module will begin counting clocksignals at substantially the same synchronized time. The microcontroller600 also includes a reset circuit 608 coupled to a voltage detector 610that receives a power level voltage from power line 507 of the circuit500. The voltage detector 610, for example, may maintain themicrocontroller in an off state until the voltage across capacitor 506reaches a certain amount. The reset circuit 608 may be used to set themaximum counter value for the timer circuit 604 to adjust the number ofelectrical clock signals that are counted before the microcontrollerawakes.

The power converter 500 and sensor 602 may form part of one sensormodule, where a backbone fiber would have a plurality of such sensormodules. Each module would receive the same optical clock signal via theoptical signal from the head end. The sync pulses on line 528 for eachsensor module serves as the timing starting point that synchronizes allthese sensor modules to each other, and to the head end sending theoptical clock signal. Each sensor modules' microcontroller 600 is resetby the sync pulses received at the interrupt circuit 606. Each timercircuit 604 then counts the number of clock signals received after thatsync pulse, which the microcontroller 600 can do in a sleep mode. Eachsensor modules' microcontroller 600 may be programmed, in firmware, tocount a different number of clock pulses before waking themicrocontroller 600, via a timer overflow interrupt. In this way, thesensor modules are time-division-multiplexed to turn on at differenttimes. The head end may be programmed to identify which sensing moduleis awake at a particular time based on the number of clock pulsescountered therein, based on the order in which the sensor module isawakened in relation to the other sensor modules on the backbone fiber,or based on the number of clock signals provided by the head end. In anyevent, the head end is able to identify which of the sensor modules onthe backbone fiber is transmitting its sensed output signal at a giventime.

The microcontroller 600 may be coupled to an input/output stage 612coupled to a sensor 614 that is positioned to sense a measurableparameter or property in the sensed region around the sensor module. Themicrocontroller 600 may power any type of sensor desired for sensing,including both optical and non-optical sensors and those of low or evenhigh power, if operated only for short periods. In an example, themicrocontroller 600 may be coupled to a tin oxide (SnO₂) hydrogen sensorfor monitoring hydrogen content in environments on an aircraft.Alternatively, a solid-state hydrogen sensor using palladium films maybe used. In any event, the examples are not limited to a particularlytype of sensor. The sensor 614 may include a light source and aphotodiode, for example. In alternative examples, such as measuringoperating conditions on a power line at remote locations, a current orvoltage sensor/detector may be used.

In the illustrated example, the signal from the sensor 614 is coupled toan amplifier 616 that may include an optional shutdown pin coupled toinput/output interface 612 of the microcontroller 600 to save power. Thesignal is then coupled to an analog-to-digital converter (ADC) 618 andback into the microcontroller 600, at serial port or bus 619, which thenprocesses the signal and uses it to control and power a laser driver 620for driving a laser 622. The laser driver 620, for example, may be amodulator and the laser 622 a VCSEL. An output 624 (e.g., a sensedoutput signal) of the laser 622 is coupled to the backbone fiber via acoupler, such as the coupler 110 or other couples described above withreference to FIGS. 1, 2A and 2B.

In the illustrated configuration, power for the devices may be turnedoff except for the voltage detector. For example, by using an operationat amplifier with a shut down pin, as the amp 616, the microcontroller600 may turn off the amp 616 when it is not needed. In fact, the sensor614, ADC 618, laser device 620, and laser 622 may be turned off when themicrocontroller 600 is asleep, leaving only the voltage detector 610 on.This ability to operate in sleep mode may substantially reduce powerconsumption. Additionally, operating the ADC 618 and laser driver 620 onthe same serial port or bus 619 provides power advantage, as themicrocontroller 600 uses less clock cycles and as the elements 619 and620 may be kept off longer.

The sensor 602 is shown by way of example. The sensor 602 may includeadditional or fewer elements. The sensor 602 may include additionalsensors as well, such as voltage or temperature sensors that can be usedto monitor and communicate sensor performance data to the head end.

To illustrate an example operation of the power converter and the sensor602, FIG. 7 illustrates an example optical signal 702 that may be sentfrom the head end 106. The clock signal 702 includes a clocking portion707, where the clock cycles occur, and a synchronization (dropout)portion 708 where a number of clock cycles have been skipped by the headend 106, and the optical signal 702 is in a continuous low state. Theclock signal portion 707 has a 50% duty cycle comprising a high cyclestate 704 and a low cycle state 706 on every clock cycle.

Line 710 represents the electrical clock signal line 522 from the powerconverter 200 to the sensor 602. The clock signal 710 is maintained highthrough the first high cycle 704. However, after the high cycle 704, thepower converter 500 is triggered to supply a negative clock signal 712to the sensor 602. The sensor 602 may count the received clock signals712 via the timer circuit 604. The sensor 602 may be separatelyprogrammed to output a sensed signal after a determined number of theseclock signals 712 have been counted. Line 714 illustrates the syncoutput line 528 for the power converter 500.

The power converter 500 has an output to provide a sync signal to themicrocontroller 600. By way of example, not limitation, a sync signal716 is sent from the power converter 500 to the microcontroller 600following the end of the first clock cycle after the dropout in theclocking 708. In the illustrated example, the sync pulse 716 occurs justafter the dropout in the clocking portion 708 has ended in accordancewith activation of the switch 414 or 530 due to the inductive kickelement of 504. Each sensor module on the backbone fiber would receivethis sync pulse 716 simultaneously, which may be used to synchronizeeach of the sensor modules.

A first output signal from a first sensor module is illustrated at line718 and includes sensed data 720. The sensor module producing the outputsignal 718 has been programmed to provide its sensed data 720 aftercounting a single clock pulse 712′, after the sync signal 716. Thesensed data 720 is only sent during a low cycle 706′ of the clock signal702 to avoid interference with the high cycle 704, as both clock signaland sensed signals are sent on the same optical fiber 106. Alternativelythe sensed data 720 may be sent during a high cycle when, for example,the sensor laser source operates at a different wavelength than that ofthe signal from the head end. A second sensor module may count adifferent number of clock signals after the sync signal 716 and producean output 722. In this example, the sensor module has been programmed tocount two clock pulses (712′ and 712″) after the sync data 716, beforefor uplinking its sensed data 724 during another low cycle 726.

The illustrated output 718 and 722 represent signals produced by thelaser 622 and are by way of example. The temporal separation betweensensed output signals from different sensor modules may be betterresolved in a time division multiplexed configuration if each sensormodule is programmed to provide an output after numerous clock signals.By way of example, FIG. 8 illustrates a sync line 802 with threeidentical clocking portions 804, 806, and 808, each representing 50clock cycles. After the first temporal region 804, a first remote sensorproduces an output signal 810. After the second temporal distance 806, asecond remote sensor produces an output 812. After the third temporalregion 808, a third remote sensor produces the output 814.

Numerous alternatives exist. Techniques and apparatuses for providing anoptical system of time division multiplexed remotely located sensormodules are described. And, while it is contemplated that each of thesensor modules could transmit a signal to a head end after a differentnumber of electrical clock signals have been counted, alternatively oneor more of the distributed sensor modules may communicate a sensedsignal at the same time. The head end may be able to resolve suchsignals based on differences in frequency, amplitude, or phase, forexample. Furthermore, although in some examples it is useful to providea sensed signal during a low state of the optical signal from the headend, some or all of the sensed signals may be transmitted during a highstate. In further alternatives, data other than sensed data may beprovided by the remote sensor module. The sensor modules, in particulartheir microcontrollers, may be programmed to provide operational data onthe sensor module, for example, data indicating whether the sensormodule is operating or the present voltage level out of the powerconverter.

The sensed output signal includes data representing a measuredparameter, such as temperature. The data may be conveyed by the strengthof the sensed signal sent, or via frequency modulation, phasemodulation, binary 1's and 0's, or other information impartingtechniques. The output signal from the sensor module could insteadrepresent an actual counter value stored in a timer. This counter valuecould be used by the head end to determine if any of the remote sensormodules lag behind others. In such examples, the head end may send areset or other instructional data signal to the remote sensor modules.Or at least the head end may identify to a user which sensor modules maybe malfunctioning. In any of these examples, multiple data types may beuplinked to the head end during a sync portion.

Numerous applications may be achieved with systems in which a head endis capable of optically powering remote sensors on a fiber, where thoseremote sensors are able to communicate a sensed signal or otherparameter back to the head end on that fiber. For example, a head endcan operate at higher power levels, but then reduce output power uponsensing a break in the fiber. The head end may detect where one or moreof the nodes are unresponsive. The head end may then either turn off theoptical power signal, or reduce the optical power down to a lower, saferlevel. Additionally, the head end may cause the sensor modules to run atlower duty cycles, using less power, by running longer intervals betweensynchronization pulses.

It is also possible to send commands from the head end to the sensormodules by, for example, altering the number of clock cycles betweendropouts, as a means of encoding transmitted data/commands. It ispossible to do such encoding, without interfering with the ability toalter the number of clock cycles between dropouts for optimization ofpower usage within the sensor modules.

Diagnostic systems may now be implemented with reduced requirements formanual inspections and reduced system downtime. In a vehicle healthmanagement system, for example, an optical sensor system may deploymultiple distributed sensors that provide in-flight diagnostic data. Insuch applications, a diagnostic system may monitor flight or vehicleconditions and generate a responsive maintenance program or protocol inresponse thereto. The diagnostic system, for example, may have a headend that collects the sensor data from various locations in a vehicleand commutes that data to a management system for algorithm-based orother decision making. The management system may be internal or externalto the vehicle, and communication may be wired or wireless, e.g., via asatellite link between a spacecraft, orbital satellite and anearth-based management system with transceiver. The management systemmay be a computer system, such as personal computer or computer networkcapable of executing code or algorithms associated with diagnosticassessments of the sensed information.

By way of example, not limitation, FIG. 9 illustrates an optical sensorsystem 900 used in an aircraft 902 that has a plurality of locations904, 906, 908 to be sensed. The locations 904, 906, 908 may representareas within the aircraft monitored for environmental conditions orother performance metrics, including flight critical data. In theillustrated example, the system 900 includes a head end 910 coupled tothree sensors 912, 914 and 916 via an optical fiber 918 for opticalpowering and data communication. The head end 910 may perform dataacquisition, receiving photonic signals from the sensors 912, 914 and916 and processes the received signals. The head 910 may perform fulldiagnostic analysis on the signals received from the remote sensors 912,914 and 916 or may communicate signals to a management system 920, suchas a computer or computer network. In the illustrated example, themanagement system 920 is external to the aircraft 902 and incommunication with the aircraft communications system or head enddirectly via a transceiver 922. By way of example, not limitation, themanagement system 920 may execute coded algorithms to perform dataanalysis such as filtering, data comparison, datacompression/decrompression, Fourier transforms, power spectral densitycalculations, and diagnosis of sensed structural component fatigue,usage, overload conditions, and/or environment exposures, depending onthe sensors deployed. In some examples, the management system 920 mayperform or be part of a predictive system that executes prognosticalgorithms based on diagnostic data. Such algorithms may be useful inpredicting crack growth, strain life, corrosion damage, or otherresidual strength and life metrics based on diagnostic data and vehicledata (e.g., payload, usage, and environmental exposure). Example systemsare described in U.S. Pat. No. 6,691,007, entitled “Vehical ConditionMonitoring System,” which is expressly incorporated herein by reference.

Although certain apparatus constructed in accordance with the teachingsof the invention have been described herein, the scope of coverage ofthis patent is not limited thereto. On the contrary, this patent coversall embodiments of the teachings of the invention fairly falling withinthe scope of the appended claims either literally or under the doctrineof equivalents.

1. An optical power converter comprising: a photodetector for producingan electrical signal; a storage circuit in parallel with thephotodetector to store at least a portion of the electrical signal; afirst chargeable switch; a second chargeable switch having a differentcharging time than the first chargeable switch; and a dual transistorswitch coupled to the first chargeable switch and the second chargeableswitch, wherein the storage circuit is coupled to the dual transistorswitch, and wherein during a storing state the photodetector suppliescurrent to the storage circuit and the dual transistor switch is in anoff state, and wherein during a driving state, the storage circuitsupplies current to switch the dual transistor switch to an on statewherein at least one of the first chargeable switch or the secondchargeable switch is in a conducting state.
 2. The power converter ofclaim 1, wherein the photodetector is a solar cell.
 3. The powerconverter of claim 2, wherein the solar cell has a PIN layerconfiguration.
 4. The power converter of claim 2, wherein the solar cellis a pie-wedge solar cell.
 5. The power converter of claim 1, whereinthe first chargeable switch comprises a first capacitor coupled to thedual transistor switch and wherein the second chargeable switchcomprises a second capacitor coupled to the dual transistor switch. 6.The power converter of claim 5, wherein the first chargeable switchcomprises at least one resistor in series with the first capacitor andwherein the second chargeable switch comprises at least one resistor inseries with the second capacitor.
 7. The power converter of claim 5,wherein the first capacitor is coupled to a first output line to providea clock signal and wherein the second capacitor is coupled to a secondoutput line to provide a synchronization signal.
 8. The power converterof claim 1, wherein the storage circuit comprises an inductor andcapacitor in parallel with the photodetector, and wherein the inductoris coupled to the dual transistor switch such that in the driving state,the first chargeable switch and the second chargeable switch receivecurrent from the dual transistor switch, and wherein in the storingstate, the first chargeable switch and the second chargeable switchreceive substantially no current from the dual transistor switch.
 9. Thepower converter of claim 1, wherein the dual transistor switch comprisestwo transistors with coupled emitters.